The present invention relates to a transmission apparatus for transmitting a signal with a plurality of career frequencies.
A conventional communication apparatus, more particularly, a transmission apparatus is explained using Japanese Unexamined Patent Publication HEI8-274734. FIG.1 is a block diagram illustrating a configuration of the conventional transmission apparatus. In a radio communication apparatus with the above configuration, input modulated signals input from input terminals 111 to 11n are respectively subjected to frequency conversion to be with different frequency bands to each other in frequency conversion sections 121, to 12n. The converted outputs are provided to power combining section 16 through respective variable attenuators 211, to 21n, to be signal-multiplexed, and the multiplexed signal is output. A part of the multiplexed output is branched, and an envelope power level is detected at level detection section 23. When the detected level L exceeds level Ls which is k times (k is about 4 to 5) the average power level of the multiplexed signal, control section 24 controls attenuators 211, to 21n to attenuate the average power level of the modulated signal power to be less than k/n time the average power level for about 1/xcex94Fo (sec) (Fo is a frequency of a band width of the multiplexed signal).
However, there are two problems described below in the conventional technology.
First, in the conventional technology, since an attenuation amount is determined based on the power level combined in the power combining section, the problem occurs that a transmission signal is output from the power combining section while the level detection section, the control section and attenuation sections are executing the processing. Therefore, it is necessary to configure an amplifier, which is provided at a latter part in the configuration, in such a manner that distortions are not generated therein even when a large peak power level is inputted thereto. Generally, the amplifier configured in such the manner has a large scale and large heating value, and a high price.
In addition, the conventional technology is considered to employ FSK signals as transmission signals. The FSK modulated signals have a single frequency during a symbol time. The frequency for the symbol normally ranges from several kHz to several hundreds kHz, which is a very long time with respect to the time 1/xcex94Fo for the attenuation described in this specification. Accordingly, the conventional technology adequately functions even when a response rate to the attenuation is slow to some degree.
However, in a PSK modulation system and a QAM modulation system, amplitudes and phases vary even during the symbol time. The rate for such variations is almost equal to the rate for the variations for power addition results. Accordingly, in the case where the response rate to the attenuation is slow, the suppression for the peak power level is not performed in time, and the power level attenuates at the time the power addition result does not exceed an expected level.
An object of the present invention is to provide a transmission apparatus capable of attenuating a peak power level prior to the signal output, without needing an amplifier configured in such a manner that distortions are not generated therein even when a large peak power level is inputted thereto.
The object is achieved by a transmission apparatus for performing an orthogonal modulation with a plurality of carrier frequencies, adding modulated signals to detect a peak power level, based on the peak power level, calculating a coefficient for suppressing an amplitude of a transmission baseband signal, and using the coefficient suppressing the amplitude of the baseband signal to be inputted to a band pass filter. It is thus possible to suppress the peak power level assuredly, and unnecessary frequencies are generated by effects of the band pass filter.